Flammable substances such as gasoline may be used and/or stored in garages and other locations where gas-fired water heaters are located. Accidents may occur when the pilot light of the water heater ignites vapors produced from flammable household substances. Vapor fires typically begin in the ignition chamber of the water heater. One approach to preventing vapor fires is to adapt the water heater so that the flame arrestor plate remains below the auto-ignition temperature of the vapor outside the chamber when a vapor fire begins inside the combustion chamber. Serious accidents may still result when the vapor fire inside the chamber heats the arrestor plate to temperatures exceeding the auto-ignition temperature of the vapor outside the chamber. Furthermore, vapor fires within the combustion chamber may irreparably damage the water heater.
The United States Patent Application having publication number U.S. 2001/0038986 A1 describes a safety shutoff for gas water heaters, in which a vapor sensor is continuously monitored by a microprocessor. When vapor is present the impedance of the sensor is changed, and the microprocessor shuts off the gas flow valve. Disadvantages of this approach are the cost and complexity of implementing a microprocessor-based control system, and performance limitations of computational methods of control.
Another approach involving vapor sensors is described in the United States Patent Application having publication number U.S. 2001/0042564 A1. The variable impedance of the vapor sensor is placed in series with the current source provided by a thermocouple to maintain a gas valve in an open position. Locating the sensor impedance in series with the thermocouple may lead to unreliable operation, as the current provided to maintain the gas valve open depends upon both the temperature of the thermocouple and the impedance of the sensor. Furthermore, once closed, the gas valve will open again according to momentary fluctuations in the sensor impedance.
United States Patent Application having publication number U.S. 2002/0134320 A1 describes use of a combustion sensitive fuse. When the fuse is exposed to flame, the gas valve is closed. A vapor combustion situation must first arise before the gas is shut off.
Other references describing various techniques to suppress vapor fires, regulate a pilot light, and detect flame are    U.S. 2001/0009144 A1    U.S. 2002/0066420 A1    U.S. Pat. No. 5,548,277    U.S. Pat. No. 5,848,586    U.S. Pat. No. 6,139,311    U.S. Pat. No. 6,390,028 B1    U.S. Pat. No. 6,401,668 B2    U.S. Pat. No. 6,412,447 B1    U.S. Pat. No. 6,435,140 B1    U.S. Pat. No. 6,474,979 B1    U.S. Pat. No. 6,508,207 B2    U.S. Pat. No. 6,554,608 B1    Europe December 1993 0 382 893 A1    Europe August 1996 0727 613 A1